Who do you think is a better man?

Better as in smarter. Morally, they're both scum.

obviously Stringer Bell, one is brainy and the other is brawny. The brawny one even says that he made an error in not changing

Although Stringer Bell was more intelligent, he was also in it more for himself only. He had D'angelo murdered knowing what it would do to the Barksdale family. He also started looking at Avon differently once he was released from jail. Avon was always loyal to Stringer. Stringer wouldnt have even been in the powerful position that he was in if it wasnt for Avon.